A topological insulator is a material that behaves as an insulator in its bulk but whose surface contains conducting states, meaning that electrons can only move along the surface of the material. On the surface of a topological insulator there are special surface electronic states that fall within the bulk energy gap and allow surface metallic conduction. Carriers in these surface states have their spin locked perpendicularly to their momentum. Therefore, at a given energy the only other available electronic states have a different spin, so that backscattering is strongly suppressed and carrier conduction on the surface occurs with high mobility, in a similar way to edge currents in a two-dimensional electron gas. The spin orientation is caused by an interaction between the electron's motion and its spin (spin-orbit coupling), and is unique to topological insulators. This spin-orbit interaction plays a role in certain compounds and alloys composed of heavy elements, such as bismuth or antimony. Therefore, three-dimensional topological insulation has been observed most often in semimetals and semimetal chalcogenides, such as antimony, bismuth antimonide, bismuth selenide, bismuth telluride, antimony telluride, bismuth tellurium selenide, bismuth antimony tellurium selenide, thallium bismuth selenide, lead bismuth telluride, and compounds and alloys thereof. A complete listing of materials that have been experimentally verified to display topological insulating properties as of May 2013 is given in Y. Ando, J. Phys. Soc. Jap. 82, 102001 (2013), which is incorporated herein by reference.
Three-dimensional topological insulators are usually described as supporting polarized spin transport at their surfaces with a spin density proportional to the current density. See Dimitrie Culcer et al., Physical Review B 82(15), 155457 (2010). As a result, devices with tunable control of spin polarized currents might be realized, which would be useful for spintronics applications. See Dmytro Pesin and Allan H. MacDonald, Nat. Mater. 11(5), 409 (2012). According to the definition of a topological insulator, transport only occurs at surfaces. Experiments showing evidence of surface transport have all conducted on homogeneous crystals or thin films. A major problem with many of these materials is the significant amount of bulk conduction, even at low temperatures. See Helin Cao et al., physica status solidi (RRL)—Rapid Research Letters 7(1-2), 133 (2013). Such parasitic bulk conduction makes it difficult to isolate surface transport phenomena and is a barrier to applications of topological insulator surface currents.
One of the most widely studied topological insulators, Bi2Se3, illustrates the difficulty of achieving low bulk conductivity. The carrier concentration and transport properties of Bi2Se3 strongly depend on the Se partial pressure during synthesis. See H. Gobrecht et al., Zeitschrift für Physik 177(1), 68 (1964); and J. Horák et al., Journal of Physics and Chemistry of Solids 51(12), 1353 (1990). For this reason, Se vacancies, which act as double donors, are assumed to explain the large native n-type carrier concentrations in Bi2Se3. A common approach to counteract the presence of Se vacancies in Bi2Se3 is to use compensation doping or anneal materials in the presence of Se vapor during crystal or thin film growth. See J. Kasparova et al., Journal of Applied Physics 97(10), 103720 (2005); and Y. S. Hor et al., Physical Review B 79(19), 195208 (2009). Using compensation doping to effectively eliminate extrinsic conduction requires careful control over p-type dopant concentrations. The lowest carrier concentrations achieved with compensation doping in conventional bulk and thin film synthesis techniques is ˜1016 cm−3, resulting in a relatively high bulk conductivity of ˜100 1/Ω2 cm. See N. P. Butch et al., Physical Review B 81(24), 241301 (2010); and Seung Sae Hong et al., Nat. Commun. 3, 757 (2012).
Therefore, a need remains for a method to synthesize topological insulators having low bulk conductivity. There is also a need to selectively dope topological insulators as a function of position. Such inhomogeneous doping is a foundational capability that may enable advanced semiconductor devices made from topological insulators.